PROJECT SUMMARY ? PROJECT 3 NOTCH signaling is aberrantly activated in GBM and is important for maintenance of GBM initiating cells, as well as angiogenesis. Therefore, therapeutic strategies that can modulate NOTCH signaling are of particular interest for GBM. Our preliminary unpublished results have uncovered that treatment with gamma secretase inhibitor (GSI) that inhibits the NOTCH intracellular domain (NICD) release and hence NOTCH activation improves virotherapy of GBM in vivo in mice bearing intracranial GBM. We have further discovered that oHSV (and miRH16 encoded by oHSV) induce increased Jagged-1 (Jag1) one of the five NOTCH ligands on infected GBM and also increases NOTCH signaling activity in uninfected tumor cells and the tumor microenvironment. Increasing evidence suggests that NOTCH activation plays a significant role in macrophage activity and polarization. Further, our data also show that blockade of oncogenic NOTCH signaling improves anti-tumor efficacy of oHSV in vivo. Thus, we hypothesize that: (a) increased Notch ligand expression in oHSV1-infected tumor cells will result in increased NOTCH activity in uninfected tumor cells (Aim 1), (b) NOTCH activity in macrophages increases tumor inflammation (Aim 2), and (c) inhibiting NOTCH activity in conjunction with oHSV1 therapy will increase efficacy (Aim 3). Thus blocking NOTCH signaling with oHSV therapy should have significant clinical and translational implications.